Method of reversal development using two electrostatic developers

ABSTRACT

Process for electrophotographic discharge area development is provided comprising uniformly electrostatically charging a photoconductive insulating layer, exposing said photoconductive layer to an original image pattern of light and shadow, said photoconductive layer being discharged in areas corresponding to the light areas of the imaging pattern and retaining charge in areas corresponding to the shadow areas of the image pattern, said charged and discharged areas of said photoconductive layer resulting in the formation of an electrostatic latent image corresponding to the original image pattern, developing said electrostatic latent image with a first developer containing a toner which is substantially invisible when viewed against the photoconductive layer, said toner having the same or opposite polarity as that of charged areas on the photoconductive layer, whereby the toner preferentially adheres to areas of said layer wherein charged and discharged areas meet, such areas exhibiting an electric field gradient, thereafter, developing said electrostatic latent image with a second developer containing a colored toner which is visible when viewed against the photoconductive layer by contacting said layer with said second developer, said toner having the same polarity as that of the charged areas on the photoconductive layer, simultaneously establishing a potential between said second developer and the photoconductive layer, thereby depositing the color toner in the discharged areas of said layer forming a reversal image of the original image pattern.

United States Patent [191 Fukushima et al.

[ 51 Aug. 26, 1975 METHOD OF REVERSAL DEVELOPMENT USING TWO ELECTROSTATIC DEVELOPERS [75] Inventors: Osamu Fukushima, Tokyo; Sadao Osawa, Saitama; Takao Komaki, Saitama; Masamichi Sato, Saitama, all of Japan [73] Assignee: Rank Xerox Ltd., London, England [22] Filed: Dec. 20, 1973 [21] Appl. No.: 426,842

Related US. Application Data [63] Continuation-in-part of Ser. No. 206,902, Dec. 10,

1971, abandoned.

3,038,799 6/1962 Metcalfc et a1. 96/l.3 3,236,776 2/1966 Tomanek 96/1 SD X 3,262,806 6/1966 Gourge 96/] R X 3,300,410 l/1967 Oliphant.... 96/1 LY X 3,560,203 2/1971 Honjo et al 96/l.3 X 3,773,507 11/1973 Sato et a1. 96/1 R Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or FirmJames J. Ralabate; Donald C. Kolasch; Ernest F. Chapman 5 7 ABSTRACT Process for electrophotographic discharge area development is provided comprising uniformly e1ectrostatically charging a photoconductive insulating layer, exposing said photoconductive layer to an original image pattern of light and shadow, said photoconductive layer being discharged in areas corresponding to the light areas of the imaging pattern and retaining charge in areas corresponding to the shadow areas of the image pattern, said charged and discharged areas of said photoconductive layer resulting in the formation of an electrostatic latent image corresponding to the original image pattern, developing said electrostatic latent image with a first developer containing a toner which is substantially invisible when viewed against the photoconductive layer, said toner having the same or opposite polarity as that of charged areas on the photoconductive layer, whereby the toner preferentially adheres to areas of said layer wherein charged and discharged areas meet, such areas exhibiting an electric field gradient, thereafter, developing said electrostatic latent image with a second developer con taining a colored toner which is visible when viewed against the photoconductive layer by contacting said layer with said second developer, said toner having the same polarity as that of the charged areas on the photoconductive layer, simultaneously establishing a potential between said second developer and the photoconductive layer, thereby depositing the color toner in the discharged areas of said layer forming a reversal image of the original image pattern.

8 Claims, 6 Drawing Figures PATENTEB M1826 I975 lfL/z FIG. la

F/G. lb

I I IZ F16. lc

METHOD OF REVERSAL DEVELOPMENT USING TWO ELECTROSTATIC DEVELOPERS This application is a continuation-in-part of US. application Ser. No. 206,902, filed Dec. 10, 1971. now abandoned.

The present invention relates to electrophotography. More particularly, this invention relates to a reversal development electrophotographic process which substantially eliminates both the formation of random pinhole-like dots in the background of an image and any edge effects.

Reversal development has heretofore been effected employing a substrate having a photoconductive layer comprising, for example, zine oxide powder in an insulating resin binder. The photoconductive layer is uiformly charged in the dark, then, an electrostatic latent image is formed thereon by exposure to an optical image such as a negative transparency. The discharged areas of the photoconductive layer are then developed with toner exhibiting the same polarity as that of the electrostatic latent image. In this manner, a reversal image of the original can be obtained.

Unlike organic photoconductors or amorphous selenium, photoconductive layers comprising a blend of a photoconductive powder in an insulating resin are molecularly non-homogeneous. Thus, upon charging, for example, by corona discharge, the electric charge imposed on the surface of said photoconductive layer is not uniformly distributed and will therefore generally produce pinhole-like domains having substantially little or no charge. One of the reasons why such pinhole-like domains of little or no charge are produced is that in such molecularly non-homogeneous areas, the electric charge is discharged. Since reversal development is, in essence, a discharge area development, the toner will preferentially adhere to areas where little or no electric charge exists; therefore, numerous black pinhole-like dots will appear, when black toner is used, on a white background which greatly deteriorates the quality of the image obtained.

Another problem in discharge area or reversal development is the occurrence of edge effects. Edge effects arise when the electrostatic latent image exhibits a wide, even charge density area. In such instances, voltage gradients exist only at the edges of the image. Thus, a very dense development occurs at the edge of the image with substantially no development within the wide areas of the latent image which exhibit a uniform charge density, since in these areas no voltage gradient exists. In positive development, wherein the toner adheres to the area where the electric charge of the electrostatic latent image exists, it is relatively easy to substantially eliminate edge effects. Generally in positive development, the edge efect can be eliminated or reduced to practically useful levels by bringing an electrode, generally referred to as a development electrode, into proximity with the electrostatic latent image thereby creating a voltage gradient across the areas of otherwise even charge density.

In reversal development however, the toner adheres to the discharged area or to areas having less charge than that of the latent image. In most instances, in discharge area development, the edge effect cannot be removed even by employing a development electrode. The phenomenon of edge effect in reversal development is manifested by very dense development just outside domains having a wide uniform charge density with very slight development beyond said areas of dense development. When a positive print is to be obtained from a negative transparency using conventional electrophotographic discharge area development, a blank frame is usually provided outside the optical image. In order to provide a blank frame in reversal development, the blank portion of the frame must be charged to a high potential so that no toner will deposit thereon. Accordingly, that portion of the frame becomes an area exhibiting a wide uniform high charge density. Thus, the periphery of the optical image just inside of the blank frame becomes very densely developed and is conspicuously noticeable. This is true particularly when the blank frame is contiguous to an area of uniform and comparatively low charge density such as in the instance of the sky, for example. In such instance, there appears a dense objectionable line on the border of the frame and the sky. When the optical image provides a narrow uniform charge density area, the edge effect is rather imperceptible and thus is generally not considered objectionable. It has been found however, that the edge effect which arises in reversal development as hereinabove described is a great obstacle in obtaining high quality photographic images. It has heretofore been considered almost impossible to completely remove the edge effect in reversal development even when a development electrode was employed.

Accordingly, it is an object of the present invention to overcome the above-described deficiencies.

It is another object of the present invention to provide a process for discharge area development which essentially eliminates pinhole-like dots appearing in background areas.

It is still another object of the present invention to substantially eliminate the edge effect which has heretofore plagued discharge area development.

These as well as other objects are accomplished by the present invention which provides a process for electrophotographic discharge area development comprising uniformly electrostatically charging a photoconductive insulating layer, exposing said photoconductive layer to an original image'pattern of light and shadow, said photoconductive layer being discharged in areas corresponding to the light areas in the image pattern and retaining charge in areas corresponding the shadow areas of the image pattern, said charged and discharged areas of said photoconductive layer result ing in the formation of an electrostatic latent image corresponding to the original image pattern, developing said electrostatic latent image with a first developer containing a toner which is substantially invisible when viewed against the photoconductive layer, said toner having the same or opposite polarity as that of the charged areas on the photoconductive layer, whereby the toner preferentially adheres to areas of said layer wherein charged and discharged areas meet, such areas exhibiting an electric field gradient, thereafter, developing said electrostatic latent image with a second developer containing a colored toner which is visible when viewed against the photoconductive layer by contacting said layer with said second developer, said toner having the same polarity as that of the charged areas on the photoconductive layer, and simultaneously establishing a potential between the second developer and the photoconductive layer, thereby depositing the colored toner in the discharged areas of said layer forming a reversal image of the original image pattern.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagramatic representation of the process steps of the invention in an embodiment of the invention wherein the polarity of the toner in the first developer is the same as the polarity of the charged areas on the conductive layer.

FIG. 2 is a diagramatic representation of the process steps of an embodiment of the invention wherein the polarity of the toner in the first developer is opposite from the charged areas on the conductive layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One method of operation of the present invention is illustrated in FIG. 1 wherein are shown a series of photoconductive layers in consecutive stages of develop ment. In FIG. 1 where the numeral 12 represents a photoconductive layer, a first toner having the same polarity as the polarity of the charged areas and a second toner having the same polarity as the polarity of the charged area on the conductive layer are deposited from respective developing baths upon the imaged photoconductive layer. In FIG. 1(a). the photoconductive layer has been exposed to an image, and dark areas of the image are indicated on the photoconductive layer by the positive charges, and light areas of the image are indicated by the absence of any charge (uncharged areas). The imaged photoconductive layer is then exposed to the first developing bath containing positively charged first toner, and particles of the first toner shown in FIG. 1(b) as having the same charge as the charged areas of the imaged layer are represented in the drawing by an encircled plus sign. The particles of first toner are deposited upon the imaged photoconductive layer where the charged and uncharged areas of said layer meet. Following the treatment of the photoconductive layer with the bath containing the liquid first developer the imaged photoconductive layer is contacted with a bath containing liquid second developer. The liquid second developer contains colored toner particles designated in FIG. 1(c) as positively charged particles, and as shown in FIG. 1(c) said positively charged colored toner particles adhere to the surface of the imaged photoconductive layer in those areas where the photoconductive layer is uncharged or discharged and adjacent those areas having adhered toner particles deposited from the liquid first developer bath when a potential is established between the second developer and the photoconductive layer. This forms a reversal image of the original image pattern.

Another method of operation of the present invention is illustrated in FIG. 2 where a series of photoconductive layers is shown in consecutive stages of development. In FIG. 2 where the numeral 14 represents the photoconductive layer, 2(a) represents an imaged photoconductive layer, the negative charges thereon representing shadow or dark areas and the uncharged areas representing light areas. The electrostatically imaged photoconductive layer is contacted with a bath containing a liquid first developer comprising colorless or white, positively charged, toner particles shown in the drawing as encircled plus signs. FIG. 2(b) represents the the.imaged photoconductive substrate after developing in the liquid first developer. Following the first developing step where the colorless or white particles of first toner are deposited upon the imaged photoconductive layer where the charged and uncharged areas of said layer meet, the imaged photoconductive substrate is treated with a bath containing liquid second developer comprising colored, negatively charged toner particles having a polarity the same as that of the polarity of the charged areas of the imaged photoconductive layer. FIG. 2(c) represents the imaged photoconductive layer after exposure of said layer to the bath containing liquid second developer and simultaneous establishment of a potential between the second developer and the photoconductive layer, and as shown in FIG. 2(c), the negatively charged toner particles adhere to the surface of the imaged photoconductive layer in those areas where the photoconductive layer is uncharged and adjacent those areas having adhered toner particles deposited from the first developer bath. This forms a reversal image of the original image pattern.

The deposition of the toner upon the imaged photoconductive layer where the charged and uncharged areas meet, varies depending upon the charge of the toner particles in the liquid first developer. Where the charge of the toner particles of the first developer is opposite the charge of the imaged photoconductive layer, the oppositely charged colorless toner particles deposit upon the photoconductive layer essentially in correspondence with the charge upon the photoconductive layer but only at the boundaries (where the charged and uncharged areas meet) as in FIG. 2(b). In the case where the charge of the toner particles of the first developer is the same as the charge on the imaged photoconductive layer, the like-charged colorless toner particles deposit upon the photoconductive layer essentially in correspondence with an uncharged area of the photoconductive layer but only at the boundaries where the charged and uncharged areas meet as in FIG. 1(b).

The process of the present invention employs the substantially invisible toner of the first developer to preferentially deposit in any pinhole-like domains of substantially no charge as well as in the areas of the electrostatic latent image wherein the electric field gradients exist, thereby preventing the colored toner in the second developer from preferentially adhering to such areas. In this manner, both pinhole-like dots and edge effects which detract from image quality are substantially eliminated. It has heretofore been generally considered undesirable in reversal development for reasons set forth below to develop an electrostatic latent image with a developer in a conventional manner since the toner preferentially adheres not only to the pinholelike domains of substantially no charge but also to the domains within the electrostatic latent image wherein electric field gradients exist. This is attributable to the fact that development occurs within the electrostatic latent image only in areas of electric field gradients; therefore an optical image faithful to the original cannot be obtained. It is only when development is ideally effected and when development with a developer is conducted to an excessive degree that these pinhole and edge effects are not obtained. In practice, however, ideal development is never obtained since the clearance between the development electrode and the surface of a latent image is finite therefore the development electrode cannot obtain the ideal limit; consequently, these defects remain. Accordingly, it is practically impossible to eliminate the edge effect completely through use of a development electrode. It has also been previously shown that alterations of development times or alterations of the charge distribution of the latent image have been ineffective in overcoming these defects.

When developing with the first developer, if the toner in the first developer adheres in excessive amounts to areas within the latent image wherein electric field gradients exist, the amount of toner from the second developer which will adhere to these areas will be reduced resulting in a reversal edge effect. Accordingly, the particular conditions employed for development with both the first and second developers in the present invention will vary depending upon the particular electrophotographic materials and developer to be employed. These conditions can be readily determined without difficulty.

The substantially invisible toner employed in the first developer of the present invention can be prepared from materials which are colorless, light colored or white natural or synthetic polymers, pigment particles, mixtures thereof and pigment particles coated with polymer materials. Illustrative polymers which can be employed are ethyl cellulose, nitrocellulose, diacetyl cellulose, triacetyl cellulose, gelatin, polyvinyl alcohol, polyvinyl acetate, polymethyl methaerylate, polystyrene, polyvinyl chloride, polycarbonate, polyethylene and the like. Suitable pigments which can be employed are, for example, barium sulfate, calcium carbonate, kaolin, aluminum hydroxide, zinc oxide, titanium oxide, zinc sulfide, lead white and the like. These pig ments can be employed alone or with a resin coating.

It is desirable that the toner employed in the first developer possesses proper insulating properties so that it will not release its own charge to the photoconductive layer when it is deposited thereon. If this charge if neutralized, the toner employed in the second developer will deposit upon the already deposited toner from the first developer during the second development operation and the objects of the present invention will therefore not be obtained. It has been shown that in some cases the charge of the toner is not lost even when bare pigment particles are used as toner.

The toner employed in the second developer can be any material heretofore employed as a toner in dry electrostatographic development. For instance, the toner can be a mixture of carbon black and a resin. Additionally, typical toners used in liquid developers such as pigment particles bound in a resin matrix can be employed.

If desired, a development electrode can be used to aid in the deposition of the first developer. However, such use is not considered necessary and is preferably not employed. Since the object of development with the primary developer is to prevent the edge effect, the preferential adhesion of the toner from the primary developer only to the edge area is desired; therefore, it is considerd preferable not to employ a development electrode which tends to overcome the occurrence of edge effects. If a bias voltage is applied during development with the first developer, even the wide areas of uniform charge density are developed to some extent. However, if the distance between the electrostatic latent image and the developing electrode is great, the effect of the bias voltage is quite small and therefore the objects of the present invention can still be accomplished even when using a development electrode in the first development operation. In the second development operation, use of a development electrode is necessary to establish a potential between the toner of the second developer and the photoconductive layer, thereby causing the toner to adhere to the discharged areas of said layer.

The following examples further illustrate the present invention. All percentages and parts are by weight unless otherwise specified.

EXAMPLE I A photoconductive substrate was prepared by vacuum evaporation forming a 25 micron thick layer of amorphous selenium on an aluminum plate. This photoconductive layer was charged by positive corona dis charge to a +200 volt surface potential. Then, the charged photoconductive layer was exposed to a 35 millimeter negative film transparency held in an enlarger. The imaged layer was then dipped in a bath containing a liquid first developer for about 20 seconds. A development electrode was not employed in this development nor was any bias voltage applied.

The first developer was prepared as follows: 1 gram of ethyl cellulose and 50 milliliters ofa 20% solution of varnish in tetrachloroethane were dissolved in 100 milliliters of tetrachloroethane. Thirty milliliters of the resulting solution was ultrasonically dispersed in one liter of kerosene. The resulting dispersed particles of toner which were formed in the first developer mixture ranged in particle size between 0.1 and 0.5 microns. These toner particles were colorless and exhibited a positive charge.

The developed photoconductive plate was removed from the bath of first developer. Residual first developer was completely removed from the surface of the photoconductive layer and then development with a second developer was conducted. The second devel oper was prepared as follows:

Carbon black (particle size 0.0l to 0.1 l0 parts microns in diameter) Soy bean oil-modified alkyd resin 25 parts Linseed oil varnish 20 parts Deculin 10 parts These components were blended in a ball mill for 40 hours; thereafter 5 grams of the blend was dropped into a mixture of 800 milliliters of kerosene and 200 milliliters of decalin with continuous agitation thereby forming a dispersion of the blend in the mixture of kerosene and decalin. This dispersion was used as the second developer. The toner particles that were thus formed were positively charged.

In the second development step, a development electrode was used and a bias voltage was also applied. The clearance between the electrostatic latent image on the photoconductive layer and the metal plate development electrode was about 1 millimeter. The space between the electrode and the photoconductive plate was filled with the second developer. Thereafter, an volt bias voltage was applied with the developing electrode being positive relative to the aluminum plate substrate of the photoconductive material. Ten seconds later the bias voltage was turned off and the developing electrode was removed. An optical image without edge effect was formed on the selenium plate. Any residual toner was removed from the plate by washing the plate with kerosene. Then while the surface of the selenium plate was still wet with kerosene, a sheet of white paper was placed upon the developed image on the plate and a negative corona discharge was applied to the surface of the paper not in contact with the plate effecting a transfer of the toner image onto the white paper. The image thus obtained exhibited neither pinhole dots nor an edge effect and exhibited an excellent quality equal to a silver halide photograph.

EXAMPLE 2 A white electrophotographic material was prepared by coating a mixture of photoconductive zinc oxide powder in an insulating resin on a sheet of paper. This photoconductive insulating layer was charged in the dark by corona discharge to a surface potential of -250 volts. Thereafter, the charged photoconductive layer was exposed to a 4X5 inch negative film transparency forming an electrostatic latent image on the photoconductive layer. After formation of the electrostatic latent image, the image was developed with a first developer which was prepared as follows:

The following materials were admixed in a ball mill for 30 hours:

White zinc oxide powder 10 parts (0.05 to 0.5 microns in diameter) Linseed oil varnish 20 parts Resin varnish l5 parts Decalin parts milliliters of the resulting mixture was dropped into a mixture of 800 milliliters of kerosene and 200 milliliters of decalin maintained under continuous agitation to form a first developer comprising a dispersion of toner in a mixture of kerosene and decalin.

Development with the first developer was conducted by passing the photoconductive plate bearing the electrostatic latent image on the surface thereof through the nip formed by a pair of metal rollers while continuously spraying the first developer into the nip of said rollers. The roller diameters were each 50 millimeters and the clearance between the rollers was 2 millime ters. The electrophotographic material was passed through the rollers at a speed of 2 centimeters per second. Since the electrophotographic material was about 150 microns thick, the clearance between the electrostatic latent image and the roller facing it was about 1.8 millimeters. After development with the first developer, the electrophotographic material was passed through the nip of a pair of squeezing rollers for re moval of any residual developer remaining on the surface thereof. The roller contacting the surface of the electrophotographic material bearing the electrostatic latent image was made of stainless steel having a mirror finish, while the squeezing roller which touched the backside of the electrophotographic material was made of rubber. Development with a second developer was conducted immediately upon the electrophotographic material emerging from the squeezing rollers.

In the second development operation, a bias voltage was applied to each of five pairs of metal rollers through which the electrophotographic material was passed. in each instance, the roller facing the electrostatic latent image on the surface of the electrophotographic material was a 20 millimeter diameter. mirror finished stainless steel roller, whereas, the roller contacting the backside of the electrophotographic material was, in each instance. a 20 millimeter diameter stainless steel roller. The nip formed by each pair of opposed rollers was maintained at 0.3 millimeters and the clearance between the electrostatic latent image on the surface of the electrophotographic material and the roller facing it was kept at 0.1 to 0.15 millimeters. The five pairs of rollers were separated from each other with a spacing of 25 millimeters therebetween. The electrophotographic material was passed through said rollers at a speed of 1.5 centimeters per second. The second developer was sprayed onto the rollers facing the electrostatic latent image. A negative bias voltage was applied to the roller which faced the electrostatic latent image; whereas, the roller facing the backside of the electrophotographic material was grounded. The bias voltage was +50 volts.

The second developer was obtained by mixing the following components in a ball mill for 50 hours.

Carbon black (0.0] to 0.1 microns 10 parts indiameter) Nitrocellulose 20 parts Resin varnish 20 parts Butylacetate 20 parts 10' cc. of the resulting mixture was ultrasonically dispersed in a solution of 800 milliliters of kerosene and 200 milliliters decalin.

After this second development, the electrophotographic plate was dipped in a volatile solvent (ISOPER E available from Esso Standard Sekiyu K.K.), rinsed and dried by warm air. No pinhole dots nor edge effects were observed in the resulting image, the quality of which was identical to the original image on a sheet of photographic paper.

EXAMPLE 3 Good results were similarly obtained when a +50 volt bias voltage was applied during development with the first developer employing the identical procedure of Example 2.

EXAMPLE 4 A white electrophotographic material was obtained by mixing photoconductive zinc oxide powder with an insulating resin. The resulting mixture was applied to white paper. The thus formed photoconductive insulating layer was charged in the dark by a negative corona discharge to a surface potential of -l50 volts. The charged photoconductive insulating layer was exposed to a transparent negative film having a size of 4X5 inches to'form an electrostatic latent image. The thus formed electrostatic latent image was developed with the first developer which was formed by dissolving one gram of ethylcellulose and 50 milliliters of a 20% solution of resin varnish in tetrachloroethane, in milliliters of tetrachloroethane. Thirty milliliters of the resulting solution was dispersed in one liter of kerosene employing ultrasonic vibrations. The resulting dispersed toner particles exhibited diameters ranging from 0.1 to 0.5 microns. The toners were colorless and exhibiteda positive charge. Development of the electrostatic latent image with the first developer was conducted by immersing the electrophotographic material containing theelectrostatic image in a bath of the first developer for 30 seconds without using a'development electrode.

After the first development. the electrophotographic material was removed from the first developer and dipped in kerosene and washed therein for 10 seconds to remove any residual developer. Thereafter. a second development operation was conducted employing a second developer which was prepared by admixing the following constituents in a ball mill for 120 hours:

Carbon black 10 parts Nitrocellulose parts Resin varnish 20 parts Butylacetate 20 parts Ten milliliters of the resulting mixture was dispersed in a liquid mixture comprising 800 milliliters of kerosene and 200 milliliters of decalin by ultrasonic vibration.

During the second development operation, a bias voltage was applied to the electrophotographic material as it ran through five pairs of metal rollers. In each pair, the roller facing the surface containing the electrostatic latent image was a mirror finished stainless steel roller having a diameter of 20 millimeters and the other roller of the roller pair which contacted the bottom surface of the electrophotographic material was a stainless steel roller having a diameter of 20 millimeters. Each roller of the roller pair was electrically insulated and the clearance between the rollers was maintained at 0.3 millimeters. The distance between the adjacent pairs of rollers which were aligned in the direction of travel of the electrophotographic material was maintained at millimeters. The electrophotographic material passed through the rollers at a speed of 1.5 centimeters per second. The second developer was supplied in the form of a spray to each of the rollers facing the surface of the electrophotographic material containing the electrostatic latent image. The roller facing the surface containing the electrostatic latent image was provided with a bias voltage of 50 volts having negative polarity. The roller facing the bottom surface of the electrophotographic material was grounded.

v a metal plate. The surfae of the electrophotographic- After the second development operation, the resulting electrophotographic material was immersed in ISOPER E (available from Esso Standard Sekiyu K. K.) and was then dried by means of heated air. The resulting image was free from edge effects.

EXAMPLE 5 Amorphous selenium was vacuum evaporated onto an aluminum plate to a coating thickness of 25 microns thereby forming an electrophotographic material. The photoconductive layer of the electrophotographic material was charged to a surface potential of +180 volts by means of a positive corona discharge. The charged electrophotographic material was exposed to a 35 millimeter negative film held in an enlarger to form an electrostatic latent image on the surface of the electrophotographic material. Thereafter the electrostatic latent image was developed by immersing the electrophotographic material in a first developer for about 20 seconds. No developing electrode was employed and therefore no bias voltage was applied. The first developer was obtained by mixing the following components in a ball mill for 80 hours:

White zinc oxide powder 15 parts (0.05 to ()5 microns) -Continued- Nitrocellulose 20 parts Resin varnish 20 parts Butylacetute l5 parts Twenty milliliters of the resulting mixture was dispersed in a liquid mixture of 800 milliliters of kerosene and 200 milliliters of decalin by continuous agitation. The resulting dispersed toner was white and exhibited a negative charge.

After the electrophotographic material was removed from the first developer, all residual first developer was removed carefully and then the resulting electrophotographic material was subjected to a second development operation. The second developer was prepared by admixing the following components in a ball mill for hours:

Carbon black 10 parts i (0.01 to 0.1 microns) Alkyd resin 25 parts Linseed oil 20 parts Decalin 15 parts Ten milliliters of the resulting mixture were dispersed with continuous agitation in a liquid mixture of 800 milliliters of kerosene and 200 milliliters of decalin. The resulting dispersed toner was black and exhibited a positive charge. I I

In the second development operation, a development electrode was employed and a bias voltage was provided. In this instance, the development electrode was material bearing the electrostatic latent image was coated with the second developer. Thereafter the coating of second developer was contacted with the development electrode to effect a clearance between development electrode and the electrophotographic material of lfmillimeter. Thereafter a bias voltage of +60 from the selenium plate to the white paper. The result- I ing image was of high quality and free fromany edge effects.

EXAMPLE 6 Employing the procedure described in Example 1, the electrophotographic material, after the first development operation, was passed through a pair of squeeze rollers instead of being washed to thereby remove excess first developer. The squeeze roller contacting the surface of the electrophotographic material bearing the electrostatic latent image was a mirror finished stainless steel roller having a diameter of 20 millimeters. The squeeze roller contacting the bottom surface of the electrophotographic material was a metal roller around which rubber having a thickness of 5 millimeters was applied thereby forming a roller having a total diameter of 20 millimeters.

The electrophotographic material was passed through said pair of squeeze rollers at a speed of 1.5 centimeters per second. Thereafter, the second development operation was conducted in the identical manner as described in Example 1.

This example illustrates that the process of the present invention can be conducted in a continuous manner on automatic equipment in a manner sufficient to prevent the possibility of the first developer from being mixed with the second developer, without the necessity of washing as in Example 1.

Although specific materials and conditions were set forth in the above exemplary processes for discharged area development in accordance with the present invention, these are merely intended as illustrations of the present invention. Various other toners, developer systems and processes such as those listed above may be substituted in the examples with similar results.

Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. Process for electrophotographic discharge area development consisting essentially of uniformly electrostatically charging a photoconductive insulating layer, exposing said photoconductive layer to an original image pattern of light and shadow, said photoconductive layer being discharged in areas corresponding to the light areas of the imaging pattern and retaining charge in areas corresponding to the shadow areas of the image pattern, said charged and discharged areas of said photoconductive layer resulting in the formation of an electrostatic latent image corresponding to the original image pattern, developing said electrostatic latent image with a first developer containing a toner which is substantially invisible when viewed against the photoconductive layer, said toner having the same or opposite polarity as that of charged areas on the photoconductive layer and having insulating properties to prevent release of its charge to the photoconductive layer, whereby the toner preferentially adheres to areas of said layer wherein charged and discharged areas meet, such areas exhibiting an electric field gradient, thereafter, developing said electrostatic latent image with a second developer containing a colored toner which is visible when viewed against the photoconductive layer by contacting said layer with said second developer, said toner having the same polarity as that of the charged areas on the photoconductive layer, simultaneously establishing a potential between said second developer and the photoconductive layer, thereby depositing the color toner in the discharged areas of said layer forming a reversal image of the original image pattern.

2. Process as defined in claim 1 wherein a potential is established between said first developer and the photoconductive layer.

3. Process as defined in claim 1 wherein said substantially invisible toner of the first developer comprises colorless, light-colored or white polymers, pigments or mixtures thereof.

4. Process as defined in claim 1 wherein the photoconductive insulating layer comprises amorphous selenium.

5. Process as defined in claim 1 wherein the photoconductive insulating layer comprises zinc oxide in an insulating resin binder.

6. Process as defined in claim 1 wherein the colored toner of the second developer contains carbon black as the colorant.

7. Process as defined in claim 1 wherein the toner of the first developer has the same polarity as the charged areas of the photoconductive layer.

8. Process as defined in claim 1 wherein the toner of the first developer has the opposite polarity as that of the charged areas of the photoconductive layer. 

1. PROCESS FOR ELECTROPHOTOGRAPHIC DISCHARGE AREA DEVELOPMENT CONSISTING ESSENTIALLY OF UNIFORMLY ELECTROSTATICALLY CHARGING A PHOTOCONDUCTIVE INSULATING LAYER, EXPOSING SAID, PHOTOCONDUCTIVE LAYER TO AN ORIGINAL IMAGE PATTERN OF LIGHT AND SHADOW, SAID PHOTOCONDUCTIVE LAYER BEING DISCHARGED IN AREAS CORRESPONDING TO THE LIGHT AREAS TO THE IMAGEING PATTERN AND RETAINING CHARGE IN AREAS CORRESPONDING TO THE SHADOW AREAS OF THE IMAGE PATTERN, SAID CHARGED AND DISCHARGED AREAS OF SAID PHOTOCONDUCTIVE LAYER RESULTING IN THE FORMATION OF AN ELECTROSTATIC LATENT IMAGE CORRESPONDING TO THE ORIGINAL IMAGE PATTERN, DEVELOPING SAID ELECTROSTATIC LATENT IMAGE WITH A FIRST DEVELOPER CONTAINING A TONER WHICH IS SUBSTANTIALLY INVISIBLE WHEN VIEWED AGAINST THE PHOTOCNDUCTIVE LAYER, SAID TONER HAVING THE SAME OR OPPOSITE POLARITY AS THAT OF CHARGED AREAS ON THE PHOTOCONDUCTIVE LAYER AND HAVING INSULATING PROPERTIES TO PREVENT RELEASE OF ITS CHARGE TO THE PHOTOCONDUCTIVE LAYER WHEREBY THE TONER PREFERENTIALLY ADHERES TO AREAS OF SAID LAYER WHEREIN CHARGED AN DISCHARGED AREAS MEET, SUCH AREAS EXHIBITING AN ELECTRIC FIELD GRADIENT, THEREAFTER, DEVELOPING SAID ELECTROSTATIC LATENT IMAGE WITH A SECOND DEVELOPER CONTAINING A COLORED TONER WHICH IS VISIBLE WHEN VIEWED AGAINST THE PHOTO CONDUCTUVVE LAYER BY CONTACTING SAID LAYER WITH SAID SECOND DEVELOPER, SAID TONER HAVING THE SAME POLARITY AS THAT OF THE CHARGED AREAS ON THE PHOTOCONDUCTIVE LAYER SIMULTANEOUSLY ESTABLISHING A POTENTIAL BETWEEN SAID SECOND DEVELOPER AND THE PHOTOCONDUCTIVE LAYER THEREBY DEPOSITING THE COLOR TONER IN THE DISCHARGED AREAS OF SAID LAYER FORMING A REVERSAL IMAGE OF THE ORIGINAL IMAGE PATTERN.
 2. Process as defined in claim 1 wherein a potential is established between said first developer and the photoconductive layer.
 3. Process as defined in claim 1 wherein said substantially invisible toner of the first developer comprises colorless, light-colored or white polymers, pigments or mixtures thereof.
 4. Process as defined in claim 1 wherein the photoconductive insulating layer comprises amorphous selenium.
 5. Process as defined in claim 1 wherein the photoconductive insulating layer comprises zinc oxide in an insulating resin binder.
 6. Process as defined in claim 1 wherein the colored toner of the second developer contains carbon black as the colorant.
 7. Process as defined in claim 1 wherein the toner of the first developer has the same polarity as the charged areas of the photoconductive layer.
 8. Process as defined in claim 1 wherein the toner of the first developer has the opposite polarity as that of the charged areas of the photoconductive layer. 